A variety kinds of vehicle power transmission devices for easy driving have been developed in an attempt to facilitate the driving of vehicles and to reduce fatigue to the drivers by automatically operating the clutch or the transmission that transmits the power of the engine to the wheels. A representative example may be a so-called AT vehicle by using a power transmission device comprising a torque converter which is a fluid transmission device and a planetary gear mechanism. One of them may be a power transmission device for easy drive which uses a transmission of the type of a parallel axis gear mechanism similar to that of a so-called manual vehicle combined with an automatic clutch, omitting the clutch operation at the time when the driver attempts to change the speed by using the speed-change lever, and has already been employed in the vehicles placed in the market. There has also been provided a power transmission device which automatically changes the gear depending upon the traveling conditions of the vehicle by using an electronic controller and an actuator for operating the transmission instead of operating the speed-change lever by the driver.
In recent years, there has been developed a power transmission device having a fluid coupling interposed between an engine and a transmission for use in vehicles equipped with a diesel engine. The fluid coupling is a fluid transmission device. Unlike the torque converter, however, the fluid coupling has no stator base and does not have a function for increasing the torque, but is simpler in structure than the torque converter.
With the fluid coupling being interposed, the vehicle can be started by utilizing slipping between the pump and the turbine in the fluid coupling particularly when the vehicle uses a diesel engine that produces a large torque in a region of low engine revolutions. Namely, this easily realizes a smooth start without requiring sophisticated clutch work which is carried out at the time of starting a manual vehicle and, at the same time, absorbs fluctuation in the engine torque during the idling and, further, reduces vibration and noise. An example of the power transmission device having a fluid coupling between the engine and the transmission has been disclosed in JP-A-2001-241546.
A vehicle power transmission device equipped with the above fluid coupling will now be described with reference to FIGS. 2 and 3. FIG. 2 is a sectional view illustrating a power transmission device between the crankshaft of a diesel engine and the transmission, and in which a fluid coupling 2 is linked at the back of the crankshaft 1 and a transmission 4 having a parallel axis gear mechanism is further linked thereto via a wet multi-plate clutch 3. The fluid coupling 2 includes a pump 21 and a turbine 22 that can be rotated independently of each other, and a casing 23 thereof is filled with an operation oil. The pump 21 in the fluid coupling is integrally coupled to the crankshaft 1 of the diesel engine by using the casing 23 and a drive plate 11. Further, an output shaft 24 of the fluid coupling 2 is coupled to the turbine 22, and a hub portion 31 of an input shaft of a wet multi-plate clutch 3 is coupled to the other end of the output shaft 24 by spline fitting, respectively. The hub portion 32 of the output shaft of the wet multi-plate clutch 3 is coupled to the input shaft 41 of the transmission 4 by spline fitting.
When the diesel engine is started to start the vehicle, the pump 21 of the fluid coupling 2 starts rotating together with the crankshaft 1, and feeds the operation oil into the turbine 22. The flow rate of the operation oil circulating into the turbine 22 from the pump 21 increases with an increase in the revolution of the diesel engine, and an increased torque acts on the turbine 22. The wet multi-plate clutch 3 connected to the fluid coupling 2 is in a connected state due to the oil pressure acting on the friction plates except when changing the speed of the vehicle. Prior to starting the vehicle, the transmission 4 is engaged with the start gear, and the vehicle is at rest since the brake pedal has been depressed. At this moment, the turbine 22, too, is at rest. As the brake pedal is released, however, the turbine 22 starts rotating, and the vehicle starts traveling via the wet multi-plate clutch 3 and the transmission 4. After the start of the vehicle, the revolution of the diesel engine further increases, and the revolution of the turbine 22 increases correspondingly. The slipping of the fluid coupling 2 decreases with the passage of time after the start, the revolution of the turbine 22 gradually increases to approach the revolution of the pump 21 (to approach the revolution of the diesel engine), and the speed ratio thereof approaches 1, i.e., the ratio of the revolution of the turbine to the revolution of the pump in the fluid coupling approaches 1.
By utilizing the fluid coupling 2 as described above, the vehicle can be smoothly started due to the slipping between the pump 21 and the turbine 22. So far as the fluid coupling 2 involves slipping, however, the power transmission efficiency never reaches 100% and the diesel engine consumes the fuel wastefully. When the vehicle starts and performs a normal traveling, therefore, it is desired to halt the function of the fluid coupling 2 and to directly couple the crank shaft 1 to the transmission 4 during the traveling at a low speed of, for example, about 20 Km/h. Therefore, the fluid coupling 2 is provided with a lockup clutch 25 for connecting the pump 21 and the turbine 22 together.
The lockup clutch 25 is placed facing the inner surface of the casing 23 that couples the crankshaft 1 to the pump 21, and is constituted by a clutch disk 26 coupled to the turbine 22 and a friction fading 27 provided on the front surface side thereof. The disconnection and connection of the lockup clutch 25 are controlled by changing over the flow passage through which the operation oil of a high pressure flows in the casing 23 of the fluid clutch 2. For this purpose as shown in FIG. 3 which is a vertical sectional view, a trochoid pump 51 for pressure-feeding the operation oil and a flow passage change-over valve 52 for changing over the flow passage of the operation oil, are mounted on a partitioning wall portion 5 at the rear part of the fluid coupling 2. The fluid passage change-over valve 52 is controlled by a lockup clutch control device 70.
The operation oil pressurized by the trochoid pump 51 flows into a chamber 28 in front of the clutch disk 26 from the passage at the central portion of the output shaft 24, flows into a chamber 29 at the back through a narrow gap at the outer peripheral portion of the clutch disk 26, and flows into an operation chamber defined by the pump 21 and the turbine 22. In this case, the pressure in the chamber 28 in front is higher than that in the chamber 29 at the back. Therefore, the clutch disk 26 is separated away from the casing 23, and the lockup clutch 25 is disconnected. When the flow is reversed by using the flow passage change-over valve 52, the pressure on the rear surface side of the clutch disk 26 becomes high and the friction fading 27 comes into engagement with the inner surface of the casing 23, whereby the lockup clutch 25 is connected, and the pump 21 and the turbine 22 in the fluid coupling 2 are directly coupled together. The fluid passage change-over valve 52 is changed over by the lockup clutch control device 70 that gradually varies the duty ratio of pulses by using a pilot valve to avoid the shock caused by sudden connection of the lockup clutch 25. Detailed constitution of the lockup clutch 25 and a control method thereof have been disclosed in the patent publication described above.
Operation at the time of starting the vehicle power transmission device using the fluid coupling with the lockup clutch will be described below with reference to FIG. 8. FIG. 8 illustrates changes in the revolution of the turbine and in the revolution of the engine (revolution of the pump in the fluid coupling) after the start of the vehicle having mounted thereon a diesel engine of the type of normal aspiration having a large displacement. When the vehicle is at rest, the engine is revolving at an idling speed of about 500 rpm, the wheels are at rest and, hence, the revolution of the turbine is zero. When the driver depresses the accelerator pedal in this state, the engine revolution increases, the torque increases, the turbine starts rotating, and the vehicle starts traveling. Thereafter, the revolution of the turbine increases accompanying an increase in the revolution of the engine, and the speed of the vehicle gradually increases. As the revolution of the turbine in the fluid coupling approaches the revolution of the engine, vehicle speed reaches a predetermined value and the revolution of the engine reaches near 1500 rpm which is a stall revolution that will be described later, the control device produces an instruction for connecting the lockup clutch, and the pump and the turbine are connected together (locked up) according to the instruction; i.e., the two rotates integrally together. At a moment when the instruction is output, the speed ratio which is a ratio of the revolutions of the turbine and the pump is nearly 0.8, and the lockup clutch can be smoothly connected.
Here, the stall revolution of the fluid coupling will be described with reference to FIG. 4. When the fluid coupling having a predetermined size and a torque capacity is combined with a natural aspiration-type engine (NA engine) and when the engine revolution is increased while the turbine in the fluid coupling remains at rest, the load torque that acts on the pump integrally coupled to the engine increases in compliance with a curve of secondary degree depending on an increase in the revolution. As for the output torque of the natural aspiration-type engine, on the other hand, the diesel engine has flat characteristics as represented by a solid line, i.e., the torque remains nearly constant despite of a change in the revolution. Therefore, the revolution is balanced at a point ● where a curve of the rated output torque of the engine meets a curve of the load torque of the pump, and the engine revolution does not increase any more. The engine revolution in this balanced state is called stall revolution. With the power transmission device having the fluid coupling with lockup clutch interposed between the engine and the transmission, in general, the lockup clutch is set to be connected near the stall revolution after the start. When the revolution of the pump increases up to the stall revolution, the speed ratio is, usually, near 0.8. Therefore, the lockup clutch is smoothly connected. Thereafter, the fluid coupling involves no slipping and assures a 100% transmission efficiency.
In the vehicle mounting the natural aspiration engine, i.e., mounting the engine which is not supercharged, the lockup clutch can be connected and the vehicle after it has started can be accelerated flawlessly if the lockup clutch is set to be connected when the vehicle speed has exceeded a predetermined value and the engine revolution has reached near the stall revolution. When a so-called turbo engine equipped with a turbo charger for charging in an attempt to increase the engine output is combined with the fluid coupling, however, it was clarified that the revolution of the engine after it is started rises and once reaches a nearly constant revolution and, thereafter, gradually increases again and creates a phenomenon in which the revolution becomes constant again at a high speed level (see FIG. 5). This phenomenon appears conspicuously in an engine having mounted thereon a turbo charger of a large capacity that increases the engine output over a wide range; i.e., the phenomenon expressing as if there exist a plurality of stall revolutions (hereinafter called “two-step stall”).
It is considered that the two-step stall stems from the output characteristics of the turbo engine represented by broken lines in FIG. 4. That is, in the turbo engine, the turbo charger at the start is not still exhibiting its ability to a sufficient degree, the pressure (boost) of the air supplied into the engine cylinders is low and, therefore, the output torque of the engine is low, exhibiting characteristics as presented by the lowermost broken line in FIG. 4. If the turbo engine is combined with the same fluid coupling as the one used for the natural aspiration engine, the revolution is once balanced at a point (mark ◯) where the lowermost broken line meets a curve of load torque of the pump; i.e., the engine revolution reaches the limit (stall revolution of the first step in FIG. 5). The turbo charger by its own nature is driven by the exhaust gas of the engine. At the start of the engine or during the low-load operation where the amount of the exhaust gas is small, therefore, its revolution is so low that its function is not exhibited to a sufficient degree. The degree of function of the turbo charger is closely related to the operating conditions of the engine and is grasped as a change in the boost in the intake pipe or as a change in the revolution of the engine. Here, the operating state of the turbo charger is referred to as “turbo charger output”.
As the revolution increases with an increase in the turbo charger output and as the output torque of the engine increases with an increase in the boost, the balancing point of the load torque of the pump in the fluid coupling shifts toward the direction of a high revolution. Thereafter, as the turbo charger output becomes steady to meet the engine operating condition, the engine revolution becomes constant at the balancing point. When the engine finally enters into the rated operating condition and the boost due to the turbo charger becomes the one of when the engine is in full-load operation, the output characteristics become as represented by the uppermost broken line which is balanced again at a point where it meets the curve of the load torque of the pump, and the engine revolution does not increase any more (stall revolution of second step in FIG. 5). In an engine which recirculates the exhaust gas in an attempt to reduce NOx in the exhaust gas (EGR) not being limited to the engine that has mounted thereon the turbo charger of a large capacity for increasing the engine output over a wide range, for example, the exhaust gas is recirculated during the low-load operation of the engine and the recirculation is halted near the full-load operation. Therefore, a difference in the turbo charger output further increases between when the engine is started and when the engine is in full-load operation.
When the fluid coupling with the lockup clutch is coupled to the turbo engine, however, it became clear that there arouses a problem concerning the timing for connecting the lockup clutch after the start of the vehicle. If, for example, a lockup instruction is produced when the engine revolution has approached the turbine revolution up to a speed ratio of about 0.8 after the occurrence of the stall of the first step like that of the natural aspiration engine, this moment is just the one when the boost increases accompanying an increase in the turbo charger output. Therefore, the output torque and the revolution of the engine increase again, connecting the lockup clutch becomes loses stability and, as a result, an extended period of time is required for the connecting or connecting the lockup clutch is accompanied by an increased shock. FIG. 9 is a graph illustrating a change in the engine revolution of when the fluid coupling is combined with a turbo engine which is highly supercharged by using a turbo charger of a large capacity and when the lockup instruction is produced at a moment when the speed ratio becomes 0.8. It will be learned that after the lockup instruction is produced, the engine revolution varies conspicuously, an extended period of time is required for connecting the lockup clutch, and the acceleration characteristics are deteriorated before and after the lockup clutch is fastened.